Semiconductor catalysis, given its oxidative and superhydrophilic nature, is known. Indeed, this technique has attracted a great deal of attention because of the wide range of potential applications for removing toxic organic an inorganic species from coated articles such as, for example, automobile windshields, house windows, building and shower doors, table tops, etc. These photocatalytic coatings also are known as self-cleaning coatings. See, for example, U.S. Publication Nos. 2007/0254164, 2007/0254163, and 2007/0128449, the entire contents of each of which are hereby incorporated herein by reference.
From the discovery of photoinduced water splitting on titanium dioxide (TiO2 or other suitable stoichiometry) electrodes in 1972, titanium dioxide has been widely studied because of its potential photocatalytic applications. When UV-light is illuminated on titanium dixode, electron and hole pairs are generated, and they reduce and oxidize adsorbates on the surface, respectively producing radical species such as OH free radicals and O2−. These radicals decompose many, if not most, organic compounds. In addition to these advantageous features, it has been observed that the surface of titanium dioxide becomes highly hydrophilic, with a water contact angle near 0 degrees under UV illumination.
The mechanism of photoinduced hydrophilicity, or photocatalytic splitting of water, is based on photogenerated electrons and holes. Electrons are generated in the conduction band, and holes are generated in the valence band. The mechanism can be generally modeled by the following:TiO2+hv→e−+h+ (surface on TiO2)Electron reaction: e−+O2→O2+− (super oxide radical)2O2+−+2H2O→2*OH +2−+O2Surface reaction: h++OH−→*OHTitanium dioxide is a good photocatalyzer in terms of reactivity, durability, safety, absorption of UV light, and scratch resistance.
One disadvantage associated with these types of semiconductors, and specifically titanium dioxide, is that given its band gap, UV light is necessary for the desired performance properties. In other words, these types of semiconductors (including titanium dioxide) require photons of energy, which often times are greater than or equal to 3.0 eV (wavelength <413 nm), to be driven. Hence, initiation of UV-illumination is required in order to activate such photocatalytic semiconductors, including photocatalytic titanium dioxide. This initiation typically takes up to a few hours depending on the light source, structure, ingredients, and texture of these coatings. Thus, at least the duration and intensity of the initiating light impacts the dosing time required for activation of the photocatalytic layer.
Thus, it will be appreciated that there is a need in the art for techniques for reducing the dosing time to enable quick activation of photocatalysts. For example, it will be appreciated that it would be desirable o reduce dosing time from a few hours to a few minutes or even seconds.
The inventors of the instant application have discovered a way of significantly reducing the dosing time to enable quick activation of photocatalysts. More particularly, the inventors of the instant application have surprisingly and unexpectedly discovered that providing a UV-reflecting underlayer results in superior reductions to the dosing time to enable quick activation of photocatalysts.
In certain example embodiments of this invention, a method of making a coated article is provided. A substrate to be coated is provided. A UV-reflecting coating is disposed, directly or indirectly, on the substrate to be coated. A photocatalytic layer is disposed over the UV-reflecting coating so that at least some of any UV light that is not initially used in the activation of the photocatalytic layer and otherwise would pass into the substrate is reflected back towards the photocatalytic layer by the UV-reflecting coating.
In certain example embodiments of this invention, a coated article comprising a substrate supporting a coating is provided. A UV-reflecting coating is disposed, directly or indirectly, on the substrate. A photocatalytic layer is disposed over the UV-reflecting coating so that at least some of any UV light that is not initially used in the activation of the photocatalytic layer and otherwise would pass into the substrate is reflected back towards the photocatalytic layer by the UV-reflecting coating.
In certain example embodiments of this invention, a method of making a coated article is provided. A substrate to be coated is provided. A UV-reflecting coating is disposed, directly or indirectly, on the substrate to be coated. A photocatalytic layer is disposed over the UV-reflecting coating so that at least some of any UV light that is not initially used in the activation of the photocatalytic layer and otherwise would pass into the substrate is reflected back towards the photocatalytic layer by the UV-reflecting coating. The photocatalytic layer comprises titanium dioxide. The UV-reflecting coating is disposed so as to reflect at least about 90% of any light useful in photocatalytic initiation coming into contact therewith. The coated article has a visible transmission of at least about 90%.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.